1. Field of the Invention
The present invention relates to a device for suppressing or preventing a spar buoy installed in the water from inclining or yawing due to tidal currents, waves, and the like.
2. Description of the Prior art
Pull-in mooring type spar buoys, which are so arranged that a buoyancy body is provided halfway on a marker mast, the bottom end of the marker mast being moored to a sinker sunk at a water bottom by a mooring device, and that the buoyancy body is pulled in the water so that the water line comes halfway of the marker mast above the buoyancy body, thus allowing the spar buoy to erect in the water by buoyancy, are less in separation length and relatively less in yawing. Therefore, they have been used in navigational aids, boring equipment, and the like.
However, the marker mast is erect over a range from nearly the water bottom to the water surface in installation, thus subject to direct effect of tidal currents and waves, such that the spar buoy may incline or yaw. When the spar buoy is used in relatively shallow waters, the spar buoy will more likely undergo such inclination and yawing.
For example, referring to a case where a mooring rope is used as the mooring device, the spar buoy is normally urged upward by buoyancy, giving rise to a possibility that the spar buoy may show two modes of yawing as shown in FIGS. 5 and 6. Mode 1 yawing (FIG. 5) is a pitching about the spar buoy's center of gravity, while Mode 2 yawing (FIG. 6) is a pitching about the mooring point. When another mooring device is used, there may occur another yawing that the spar buoy turns about its center axis.
The natural period of these types of yawing of the spar buoy, differing depending on water depth (i.e. size of the spar buoy) and the mooring device's length, is, for example, 2-3 sec. for the pitching in Mode 1 and 18-20 sec. for the pitching in Mode 2, in the case of a 20 m water depth and a 2 m mooring rope length.
The shallower the water depth, the shorter the natural period of Mode 2, while the shorter the mooring rope length, the shorter the natural period of Mode 1.
The Mode 1 pitching (FIG. 5) will not occur when the mooring device used is a direct-coupled mooring type such as universal joint (rope length: 0). Conversely, it may occur when such a mooring device as a short chain is used in order to facilitate control of water depth or to facilitate installation and relocation for use of boring.
Also, the spar buoy may incline when undergoing a resistance to water currents such as tidal currents. Slow water currents, which cause the spar buoy to less incline, are negligible. However, a little faster water currents will cause a greater inclination, such that a beacon-use spar buoy may result in an inclined lamp and therefore in its deteriorated function as a beacon, or that a boring-use spar buoy may result in an inclined working scaffold with worsened workability.
Also, when a water current impinges upon the marker mast or the columnar portion of the buoyancy body, there may occur Karman vortices or flutters. Incident cycle of Karman vortices, differing depending on the diameter of marker mast and the flow velocity, is 3 to 4 sec. for a flow velocity of 3 knots and a marker mast diameter of 0.6 to 0.8 m. The flutters, on the other hand, will occur at the natural period of Mode 2. These phenomena will act on the spar buoy as a vibromotive force. Waves also act as a vibromotive force.
Accordingly, when the natural period of yawing of the spar buoy approaches the period of waves or the incident period of Karman vortices due to water currents, both the waves and the water currents being external forces, there will occur Mode 1 (FIG. 5) and Mode 2 (FIG. 6) yawing, which may impair the function as a spar buoy.
A means for suppressing the Mode 1 yawing (FIG. 5) out of the possible two types of yawing of the spar buoy may be to attach totally four sheets of resistance plates a to the marker mast in such a way that they are shaped into a planar cross (see FIG. 7). This arrangement allows the spar buoy to be prevented from Mode 1 yawing and besides suppressed to be inclined.
On the other hand, a means for suppressing the Mode 2 yawing (FIG. 6) may be to attach totally four sheets of resistance plates to the marker mast so that they are shaped into a planar cross, as in the above case, at a portion away from the center of yawing, for example underneath the draft point of the marker mast. With such an arrangement, although the damping force becomes larger, yet the spar buoy is more likely subject to grain movement due to waves, resulting in an increased possibility of yawing. Moreover, water-current resistance applies also to the resistance plates attached to the marker mast, causing the spar buoy to be further inclined. Thus, this means cannot be said proper for the suppressing means.
In particular, spar buoys installed in the seas shallower than a 20 m water depth become smaller in natural period of Mode 2 yawing (FIG. 6), such that it approximates to wave periods of design conditions (e.g. 8-12 sec.), entering the resonance range. However, there has been available no effective means for suppressing yawing in such a case up to now, which has been a problem.
Also, there has not been found out a way for suppressing or preventing the spar buoy's inclination due to water currents. Therefore, up to now, it has been considered impossible to provide such spar buoys as are subject to less inclination to both waves and currents.